100 Mammalia. Circulatory System, 



The Arteries. In the Mammalian class the Aorta bends 

 over the left bronchial tube : the chief primary branches of 

 the Arch are given off at a little distance from its origin in 

 variable order. But before forming this arch, it gives off 

 close to its origin generally two (' coronary ') arteries for the 

 heart. The Aorta in some diving animals forms near to its 

 exit from the heart a considerable dilation : and so also some- 

 times the Pulnionary artery. There are four principal 

 varieties of form in which the vessels leave the arch. (1.) 

 The Human type, viz., a right arteria innominata, from which 

 the right * carotid ' and right ^ subclavian ' are given off ; the 

 left ' carotid ' and left * subclavian ' having each a separate 

 origin. (2.) The Bat t}^e, viz. two arteriao innominata^, each 

 giving off the * carotid ' and * subclavian ' of its own side. 

 (3.) The Feline type, mz. one arteria innominata giving off 

 both * carotids' and the right * subclavian' ; the left subclavian 

 having a separate origin. (4.) The Ruminant type, in which 

 the single arteria innominata gives off both carotids, and both 

 subclavian arteries. The great Arteries of the posterior 

 extremities are in most Mammals derived from a single* 

 trunk on each side the common iliac artery. Retia Mirahilia 

 are found in various situations, and are constant characteristics 

 of some genera : they have been found in the Skull, Orbit, 

 and Nasal cavity, in the Thorax, round the Spinal cord, in 

 the Arm, and in the Leg. 



The Veins. The major part of the Yenous sj^stem, after 

 the vessels have gained, in returning from the capillary area, 

 a conspicuous size, is furnished with valves f which occur 

 usually in pairs. As a general rule valves are wanting in 



* In Birds they are derived from two primary branches of the aorta, one cor- 

 responding with the external iliac and femoral, the other with the internal iliac 

 and ischiadic arteries. 



t No valves are met with in the veins of Reptiles and Fishes, and not many 

 in those of Bii-ds. 



